The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus using a developing liquid, a developing device therefor, and a program recording medium. More particularly, the present invention relates to an image forming apparatus of the type developing a latent image formed on an image carrier with a developing liquid or an image forming substance contained therein, a developing device therefor, and a program recording medium.
It is a common practice with an image forming apparatus of the type described to electrophotographically form an image by the following procedure. A data writing unit writes image data on the surface of an image carrier uniformly charged by charging means. As a result, a latent image corresponding to the image data is electrostatically formed on the image carrier. An image forming substance contained in a developing liquid fed from a developing device develops the latent image to thereby produce a corresponding visible image. The visible image is transferred from the image carrier to a paper or similar recording medium fed from, e.g., a cassette. After a fixing unit has fixed the image on the paper, the paper is driven out of the apparatus to a tray. After the image transfer, cleaning means removes the developing liquid and image forming substance left on the image carrier. Subsequently, discharging means discharges the image carrier to thereby prepare it for the next image forming cycle.
The above image forming apparatus is operable with a developing liquid consisting of a carrier liquid and toner, i. e., an image forming substance. Japanese Patent Laid-Open Publication No. 7-209922, for example, discloses an image forming apparatus using a developing liquid having viscosity of 100 mPaxc2x7S to 10,000 mPaxc2x7s for developing a latent image formed on a photoconductive element or image carrier. Specifically, a developing device included in the apparatus includes a developer carrier implemented as a developing roller or a developing belt. While the developing liquid is deposited on the above roller or belt in a thin layer, a prewetting liquid is applied to a latent image on the photoconductive element. Toner contained in the thin layer is caused to electrostatically migrate toward the latent image in the carrier liquid and presetting liquid (electrophoresis), thereby forming a toner image. As a result, a sharp image is transferred from the photoconductive element to a paper or similar recording medium with high quality. The above document teaches that the presetting liquid applied to the photoconductive element prevents the toner from depositing on the non-image area of the element and disturbing the image.
The developing liquid may be implemented as liquid ink containing dyestuffs or similar image forming substance, as taught in, e.g., Japanese Patent La id-Open Publication No. 48-16644. Japanese Patent Laid-Open Publication No. 50-99157, for example, proposes an image forming apparatus capable of forming an image with silicone oil or similar dielectric open fluid and liquid ink having a greater adhering force than the dielectric open fluid. The dielectric open fluid is applied to a charge holding surface, or image carrier, forming an open layer. At the same time, the dielectric open fluid is applied to the surface of an ink applying member or liquid carrier in order to form an open layer, and then the liquid ink is applied thereto. During development, the charge holding surface and ink applying member are caused to face each other while sandwiching the open layer, liquid ink, and open layer. Subsequently, the charge holding surface and ink applying member are moved away from each other. causing the intermediate liquid ink to electrostatically adhere to the open layer of the charge holding surface. The ink deposited on the open layer develops the latent image. Because the open layer of the ink applying member has a smaller adhering force than the ink, the ink does not remain on the ink applying member; rather, the open layer migrates toward the charge holding surface together with the liquid ink over at least part of its thickness.
Some liquids have a viscosity characteristic dependent on a shearing force, as well known in the art. This kind of liquid sequentially reduces its viscosity up to a saturation level when subjected to a shearing force derived from, e.g., agitation. When the liquid is left without any shearing force acting thereon, the viscosity sequentially increases toward a saturation level. Many of viscous developing liquids containing dense toner in a carrier liquid have this kind of characteristic.
The inventors found by researches and experiments that various problems arose when a developing liquid of the type described was applied to any one of the conventional image forming apparatuses. For example, when liquid ink whose viscosity is dependent on a shearing force is applied to the apparatus taught in the above Laid-Open Publication No. 48-16644, a ripple having a sufficient amplitude cannot occur in the ink having been left unused and therefore having increased viscosity. It is therefore likely that the ink and photoconductive element cannot sufficiently contact each other. It follows that image density is apt to be short before the ink left unused over a long period of time has its viscosity sufficiently lowered by, e.g., agitation.
Assume that the developing liquid of the kind described is applied to the apparatus disclosed in Laid-Open Publication No. 7-209922. Then, toner contained in the liquid left unused and increased in viscosity migrates at a lower speed based on electrophoresis than toner contained in the liquid lowered in viscosity by a shearing force. The resulting short deposition of the toner makes image density short. Moreover, the toner failed to migrate remains on the non-image area of the photoconductive element, contaminating the background of an image. In addition, it is difficult to separate the portions of the above liquid corresponding to the image area and non-image area, respectively, from each other due to tacking, causing the edges of an image to appear blurred and thereby degrading the sharpness of the image. Sharpness is also degraded when a developing liquid whose viscosity characteristic is dependent on a shearing force is applied to the above apparatus.
Technologies relating to the present invention are also disclosed, in, e.g., Japanese Patent Laid-Open Publication Nos. 7-334004 and 11-223997.
It is therefore an object of the present invention to provide an image forming apparatus allowing a minimum of short image density, background contamination and short image sharpness to occur despite the use of a developing liquid whose viscosity characteristic is dependent on a shearing force, a developing device therefor, and a program recording medium.
In accordance with the present invention, an image forming apparatus for depositing a thin layer of a developing liquid or an image forming substance contained therein on a latent image formed on an image carrier to thereby develop the latent image includes a liquid storing portion for storing the liquid, a liquid carrier movable while conveying the liquid deposited thereon, and a first agitating member for agitating the liquid stored in the liquid storing portion. Before the liquid carrier starts being driven for developing the latent image, the agitating member is caused to start agitating the developing liquid.
Also, in accordance with the present invention, in a developing device for depositing a developing liquid on a liquid carrier in the form of a thin layer, causing the thin layer to contact an image carrier included in an image forming apparatus, and depositing the thin layer or an image forming substance contained therein on a latent image formed on the image carrier to thereby develop the latent image, a thin layer contact member contacts the thin layer formed on the liquid carrier at a position upstream of a position where the liquid carrier and image carrier contact each other in a direction in which the liquid carrier is movable.
Further, in accordance with the present invention, in a program recording medium for mechanically recording a control program applicable to a control unit included in an image forming apparatus including a liquid storing portion for storing a developing liquid, a liquid carrier movable while conveying the liquid deposited thereon, an agitating member for agitating the liquid in the liquid storing portion, an image carrier for forming a latent image thereon, and the control unit for controllably driving the liquid carrier and agitating member on the basis of the control program, the developing liquid deposited on the liquid carrier in a thin layer or an image forming substance contained therein depositing on the latent image to thereby develop the latent image, the control unit stores the control program for starting driving the agitating member before starting driving the liquid carrier for developing the latent image.